Collada export breaks bones
After googeling hours and hours i found out, Blenders Collada exporter is just buggy. For everyone who wants to use Blender and Assimp together i recommend the FBX file format. Both, Blender and Assimp do support it, you can configure Blenders FBX exporter to use the correct Coordinate-System (-z forwards, y upwards) and it even supports exporting all animations on the animation-stack as individual animations.